GSM Logical Channels and Frame Hierarchy

GSM specifies two basic groups of logical channel, they are the Traffic Channels and the Control Channels :

Traffic Channels (TCH): The GSM network uses a TCH to transmit user data (e.g. Voice). There are two basic TCHs and they are defined as Full Rate TCH (TCH/F) and Half Rate TCH (TCH/H). A TCH/F has a data rate of 28.8 kbit/s, whereas the data rate for TCH/H is 14.4 kbit/s. At the beginning of GSM standardisation the voice codecs only required 13 kbit/s, whereas the remaining capacity of TCH/H was used for error correction (TCH/FS). However, newer codecs allow for better voice coding and can make use of TCH/H. Making use of these TCH/Hs doubles the capacity of the GSM system for voice transmission, but reduces speech quality. For data transmission there are many different traffic channels, for example TCH/F4.8 for 4.8 kbit/s, TCH/F9.6 for 9.6 kbit/s, and in the newer specifications TCH/F14.4 for 14.4 kbit/s. It should be noted that that these logical channels differ in their coding and their level of error correction. However data throughput is < 2Kb/s.

Control Channels (CCH): The GSM network makes use of many different CCHs, for they control medium access, allocation of traffic channels or the mobility management. Three main groups of CCHs have been defined, each has their own sub-channels.

Broadcast Control Channel (BCCH): The BCCH is used by the BTS to signal all MSs within a cell. The sort of information transmitted in this channel is such as the cell identification, options available within this cell (frequency hopping), and the available frequencies within the cell and in neighbouring cells, known as the neighbour list. In addition to this, the BTS sends information about frequency correction using the FCCH (Frequency Correction Channel), and also information about the time synchronisation via the SCH (Synchronisation Channel). Both of these channels, the FCCH and the SCH are sub channels of the BCCH.

Common Control Channel (CCCH): The CCCH is the channel where all information about connection set up between the MS and the BS is exchanged. For calls toward the MS, the BS uses the PCH (Paging Channel). However if a MS wants to set up a call, it uses the RACH (Random Access Channel) to send data to the BTS. The RACH implements multiple access to all MSs within a cell, and all MSs may access this channel. This however is where collisions may occur between MSs in the GSM system, so the BTS uses the AGCH (Access Grant Channel) to signal an MS that it can use the TCH or SDCCH for further connection set up.

Dedicated Control Channel (DCCH) : The previous two channels (BCCH and CCCH) are unidirectional, the DCCH and it’s sub channels are bi-directional. As long as an MS has not established a TCH with the BTS, it uses the SDCCH (Stand-alone Dedicated Control Channel) with a low data rate (782 bit/s) for signaling. The signaling is generally made up of authentication, registration and/or other data needed to set up the TCH. Each TCH and SDCCH has a SACCH (Slow Associated Dedicated Control Channel) associated with it. This channel is used to exchange system information such as the Channel Quality and the signal power level. Finally, if more signaling information needs to be transmitted and a TCH is already existing, GSM uses a FACCH (Fast Associated Dedicated Control Channel). The FACCH uses the time slots which are otherwise used by the TCH and is necessary, especially in the case of handovers where the BTS and the MS have to exchange larger amounts of data, in a smaller amount of time, known as layer 3 messages (not ISO/OSI reference model).